The present invention relates to a method for quadrature modulation and digital-to-analog conversion, and to a system for quadrature modulation and digital-to-analog conversion.
More specifically, the invention relates to an entirely new type of transmitter architecture for the transmission of information-carrying signals based on a digital-to-analog conversion directly on an intermediate frequency. Compared to conventional solutions, the invention will mean considerable simplification of the equipment necessary to achieve a transmitter of this type.
The principle is based on quantizing the signal, usually to 1 bit per sample, combined with noise shaping and quadrature modulation with exactly one quarter of the sampling rate used, before the digital-to-analog conversion is carried out at a very high frequency. After analog filtering, a ready modulated signal on a fixed intermediate frequency will be obtained, which subsequently may be very easily mixed down to the desired frequency, if necessary. This requires that the sampling rate used is at least as high as four times the desired frequency.
In conventional analog transmitter architectures it is usual to use three steps to modulate a signal to a desired frequency. Once the signal has first been quadrature modulated to a low intermediate frequency, it is mixed up to a high intermediate frequency, before it is finally mixed down to the desired frequency.
One of the conventional methods for carrying out digital-to-analog (and analog-to-digital) conversion of a low-pass limited signal (e.g., an audiosignal) is to use xcexa3xcex94modulation of the signal so that it is represented by 1 bit per sample, at the same time as the introduced quantization noise is shaped spectrally so that most of this noise ends up outside the frequency band of interest. To achieve this noise shaping to a sufficient degree, oversampling is used; i.e., a higher sampling rate than is strictly necessary according to Nyquist. One of the major advantages of this technique is that 1 bit sample values can be very easily and accurately converted from digital to analog form (or vice versa).
It is also known that an already oversampled signal can be interpolated very easily by a simple repetition of sample values, without this impairing the signal to any significant degree. Note, however, that although due to the oversampling the pass band is almost unaffected by the error made, the repeated spectra may contain more energy than desired.
Furthermore, it is common to use two multipliers for performing digital quadrature modulation of a complex baseband signal to a carrier frequencyxe2x80x94one multiplier for the in-phase signal (I) and one for the quadrature-phase signal (Q). In the particular instance of the sampling rate used being exactly four times (optionally twice) the desired carrier frequency, all the samples of the carrier frequency signal will have the values 1, xe2x88x921 or 0, which means that the multipliers can be replaced by simple logic. This is also used in known technology.
It is an objective of the present invention to simplify substantially the equipment necessary to obtain a digital quadrature modulator and digital-to-analog converter. This is accomplished by a method of the type mentioned above, the characteristic features of which are set forth in claim 1, and by means of a system of the type introduced above, the characteristic features of which are set forth in claim 3. Additional features of the invention are set forth in the other dependent claims.
The proposed solution is based on performing digital-to-analog conversion of a digitally modulated signal at a very high carrier frequency which is equal to exactly one quarter of the sampling rate through the digital-to-analog converter, optionally succeeded by an analog frequency conversion to the desired frequency if this is different from the digitally represented carrier frequency.
The invention provides a method for quadrature modulation of a complex baseband signal represented in digital form to an analog carrier frequency which is directly linked to the sampling rate. This signal can, if so desired, be further converted to another (usually lower) carrier frequency by known analog techniques. It is immaterial for the invention whether the complex baseband signal contains analog or digital information. An appropriately oversampled version of the accurately represented input signal, which is a complex baseband signal represented by the signal components I and Q, is quantized, usually to 1 bit per sample, using one of the known per se methods for shaping the quantization noise, e.g., xcexa3xcex94 modulation. The degree of necessary oversampling is determined primarily by the particular order of the noise shaping which is used, in conjunction with the requirements with respect to noise characteristics in the system concerned.
Since the input signal has already been oversampled to a considerable degree, the sampling rate for the quantized versions of the signal components I and Q may, if desired, be further increased by a simple repetition of samples. Due to the oversampling, this simple manner of increasing the sampling rate further will have a minimum effect on the signal bandxe2x80x94at the same time as the repeated spectra will also be attenuated. If, however, the requirements are more stringent, the errors may easily be corrected.
The final version of the baseband signal thus consists of two streams of quantized (typically 1 bit) sample values, where the sampling rate is many times greater than twice the highest frequency which occurs in the signal. When this baseband signal is quadrature modulated, a carrier frequency is used which is equal to exactly one quarter of this sampling rate. If an amplitude equal to 1 is selected for the carrier frequency signal, it can be represented by the values 1, xe2x88x921 and 0, so that all multiplications are replaced by simple logical operations.
But even more important is the fact that the quadrature modulated signal does not now require any increase in the number of bits per sample. Thus, the quadrature modulated signal is represented by the same number (e.g., 1) of bits per sample without any form of further error or noise being introduced by the quadrature modulator. It should also be mentioned that in the quadrature modulation, every second sample of I and every second sample of Q is to be multiplied by 0. This means that the signal components I and Q do not need to be interpolated to 4 times the desired carrier frequency, but only to twice this frequency, and I and Q will each only have to be represented by a sampling rate which is half the rate used for representing the quadrature modulated signal.
Finally, the quadrature modulated signal is transmitted through a digital-to-analog converter followed by a bandpass filter. It is important that this filter suppresses to a significant degree all signals outside the frequency band of the working signal, because the noise shaping process introduces a great deal of noise in these ranges. In addition, this filter will reduce the remains of any repeated spectra to a very low level.